The presence of U 4+ in some of the uranyl minerals and synthetics examined may be related to the conditions during their formation. The presence of U 4+ on the surface of uranyl minerals can be detected by the shape of the U 4f 7/2 peak, and the occurrence of the U 5f peak and satellite peaks belonging to the U 4f 5/2 peak. The second trend correlates with an increase in the covalency of the U O bonds with increase in Lewis basicity of the anion group, which results in a shift of the electron density from O to U.

The first observation is a consequence of an increase in the bond-valence transfer from the interstitial species (cations, H 2O) groups to the O-atoms of the uranyl-groups, which results in an electron transfer from O to U 6+. The average binding energies of the U 6+and U 4+ bands shift to lower values with (1) incorporation of divalent cations and (2) increase in the Lewis basicity of the anion group bonded to U. The results suggest that these minerals can be divided into the following four groups: (1) Uranyl-hydroxy-hydrate compounds with no or monovalent interstitial cations (2) Uranyl-hydroxy-hydrate minerals with divalent interstitial cations (3) Uranyl-oxysalt minerals with ( TO n) groups ( T = Si, P, and C) in which all equatorial O-atoms of the uranyl-polyhedra are shared with ( TO n) groups (4) Uranyl-oxysalt minerals with ( TO n) groups ( T = S and Se), in which some equatorial O-atoms are shared only between uranyl polyhedra. The occurrence and binding energies of the U 6+, U 5+ and U 4+ bands in the U 4f 7/2 peak of 19 uranyl minerals of different composition and structure were measured by XPS.

Xps peak fitting broad peakfwhm